1. Field of the Invention
The present invention relates to a torque sensor for non-contact-detecting steering torque generated in a rotating shaft of a steering wheel of a vehicle, and more particularly to a torque sensor of a type, which has impedance which is changed in accordance with the generated steering torque, which is capable of easily correcting deviation in the voltage in a neutral state which is caused from a dimensional error, an assembling error or an output error of an electronic element and which is arranged to be operated in a manner different from that during the detection so as to detect breakdown of the elements. Further, the present invention also relates to a torque sensor incorporating a bridge circuit including a pair of coils having impedances which are changed in opposite directions in accordance with generated torque and a pair of electric resistors so as to detect the torque in accordance with transient voltage generated in the connection portion between the coils of the bridge circuit and the electric resistors and structured such that an abnormality of short circuit of the pair of the coils is detected.
The present application is based on Japanese Patent Applications No. Hei. 9-190121 and Hei. 10-292765, which are incorporated herein by reference.
2. Description of the Related Art
An electric power steering apparatus for assisting a steering apparatus for an automobile or a vehicle uses rotational force of a motor. The rotational force of the motor for the assistance is, through reduction gears, transmitted to a steering shaft or a rack shaft by a transmission mechanism including gears or a belt. The structure of a usual electric power steering apparatus will now be described with reference to FIG. 15. A shaft 2 of a steering wheel 1 is connected to tie rods 6 of wheels, which must be steered, through a reduction gear 3A, universal joints 4a and 4b and a pinion-and-rack mechanism 5. The shaft 2 is provided with a torque sensor 100 for detecting the steering torque of the steering wheel 1. A motor 20 for enhancing the steering force of the steering wheel 1 is connected to the shaft 2 through a clutch 21 and a reduction gear 3A. A control unit 200 for controlling the power steering apparatus is supplied with electric power from a battery 14 through an ignition key 11. The control unit 200 calculates assist-steering instruction value I for-the assist instruction in accordance with steering torque T detected by the torque sensor 100 and vehicle speed V detected by a vehicle-speed sensor 12. In accordance with the calculated assist-steering instruction value I, the control unit 200 controls an electric current which must be supplied to the motor 20. The clutch 21 is controlled by the control unit 200 so as to be turned on or off. In usual operation state, the clutch 21 is turned on (connected). If the control unit 200 determines that the steering apparatus is out of order, or if power supply from the battery 14 is interrupted by the ignition key 11, the clutch 21 is turned off (disconnected).
The control unit 200 comprises a CPU. A general function of a program which is executed in the CPU is as shown in FIG. 16. Steering torque T detected and input by the torque sensor 100 is supplied to a phase compensator 201 so that the phase of the steering torque T is compensated in order to improve the stability of the steering system. Steering torque TA having the compensated phase is supplied to an assist-steering instruction value calculator 202. Also vehicle speed V detected by the vehicle-speed sensor 12 is supplied to the assist-steering instruction value calculator 202. In accordance with the supplied steering torque TA and the vehicle speed V, the assist-steering instruction value calculator 202 determines assist-steering instruction value I which is a required control value of an electric current which is supplied to the motor 20. The assist-steering instruction value calculator 202 is provided with a memory 203. In the memory 203, the assist-steering instruction value I corresponding to the steering torque is stored such that the vehicle speed V serves as a parameter which must be used in an operation for calculating the assist-steering instruction value I which is performed by the assist-steering instruction value calculator 202. The assist-steering instruction value I is supplied to a subtractor 200A and a differentiation compensator 204 of a feed-forward system in order to raise the response speed. Deviation (Ixe2x88x92i) in the subtractor 200A is supplied to a proportional compensator 205. An obtained proportional output is supplied to an adder 200B and an integrator 206 in order to improve the characteristic of a feedback system. Also outputs of the differentiation compensator 204 and the integrator 206 are supplied to the adder 200B so as to be added to each other. Current control value E, which is a result of addition performed by the adder 200B, is, as a motor rotating signal, supplied to a motor rotating circuit 207. Motor current value i is detected by a motor-current detection circuit 208 so that motor current value i is fed back to the subtractor 200A.
The torque sensor 100 of the above-mentioned power steering apparatus may be, for example, a torque sensor disclosed in Japanese Patent Publication No. Sho. 63-45528. In this torque sensor, two cylindrical bodies are fitted coaxially in such a manner as to rotate relatively in response to a torque generated at a shaft, wherein a long groove and teeth are alternately formed in an axial direction on the outer cylindrical surface of the inner cylindrical body while a cut is formed on the outer cylindrical body in such a manner that the overlap amount may vary according to the relative rotation between the cylindrical bodies and wherein a coil is provided so as to cover the outer cylindrical body. The torque generated on the shaft can be detected through measurement of the impedance of the coil since the impedance of the coil varies when the overlapping amount of the groove and the cut is varied by changing relative rotation position of the two cylindrical bodies.
It is certain that the above-described torque sensor is able to detect the torque generated at the shaft in response to the variation in the coil impedance. In the above-described torque sensor, however, an oscillator for oscillation of accurate sine-wave alternating current is needed for a highly accurate torque sensor since the coil is driven by a high-frequency alternating current. This causes a problem of high production costs since a large number of electronic parts which respectively require a high accuracy are needed. Another problem is that since the coil is driven by a sine-wave alternating current whereas the coil is actually driven with application of offset voltages to unify the actual direction of the current (single-side supply power drive), the structure is uneconomical with an extremely large current consumption and the large current consumption results with a large amount of heat generation.
The torque sensor encounters deviation of output voltage from a predetermined neutral voltage for a controller when input torque is zero owning to assembling error of a sensor element, such as a shaft or tolerance of en electronic part in a signal processing system. Therefore, the output voltage must always be adjusted. The adjustment of the voltage is, however, performed by adjusting the position in the torque sensor portion. Thus, a complicated operation must be performed. Since the adjustment accuracy depends on the reliability of the fixing method, movement of a fixed element causes a risk of self-steer to occur. Also A/D reference voltage for determining a predetermined voltage for the controller has tolerance. Therefore, even if the neutral voltage for the torque sensor is accurately set to a predetermined level, there is apprehension that the controller erroneously recognizes the shift of the neutral voltage from the predetermined level in spite of a fact that the neutral voltage is accurately set to the predetermined level. For example, a magnetostrictive sensor disclosed in Japanese Patent Publication No. Hei. 1-173843 comprises a memory so as to maintain the balance between two coils so that initial deviation of the sensor from the neutral level is corrected. However, the foregoing sensor cannot detect an abnormal state. To improve the reliability of the torque sensor, a plurality of sensors have been disposed to always compare values detected by the plural sensors so as to detect an abnormal state and/or prevent malfunction in accordance with change in the difference. However, there arises a problem in that the plural sensors raise the cost and a complicated detection system is required.
Further, U.S. Pat. No. 5,811,695 is incorporated herein by reference for an explanation of another type of the torque sensor. The disclosed torque sensor has a structure that a bridge circuit is constituted by a pair of coils and a pair of electric resistors. Moreover, the connection portion between the pair of the coils is connected to a power source through a coil operating circuit. The coil operating circuit incorporates a transistor which is a switching device common to the pair of the coils. A control voltage varying in a rectangular wave is applied to the gate of the transistor. The difference in the output voltage (transient voltage) allowed to appear in the portion in which the coils and the electric resistors are connected to each other is detected by a differential amplifier. In accordance with an output produced by the differential amplifier, the torque is detected.
In the above-mentioned disclosure, an abnormality monitoring portion is provided which detects an abnormality in accordance with one of the output voltages of the bridge circuit. The abnormality monitoring portion adjusts the neutral voltage of the differential amplifier in accordance with whether an instantaneous level of one of the output voltages of the bridge circuit satisfies an appropriate range. Thus, the abnormality monitoring portion controls the output voltage of the differential amplifier. As a result, a controller, to which the output voltage of the differential amplifier is applied, is able to detect an abnormality of short circuit or disconnection of both of the coils. The controller is structured in such a manner as to detect an abnormality of short circuit or disconnection of either of the coils. Note that the short circuit of the coil is a phenomenon that the coil and the ground are connected to each other.
The torque sensor disclosed as described above is applied to an electric power steering apparatus for a vehicle. If an abnormality of the foregoing type is detected, the controller interrupts control which cause assist steering torque to be generated. Thus, undesirable generation of assist steering torque can be prevented.
It is certain that the disclosed torque sensor is able to detect an abnormality of short circuit or disconnection of both of the coils or either of the coils and an abnormality of short circuit or disconnection of either of the coils.
As a result of an energetic investigation of the inventors of the present invention, a fact is found that the disclosed torque sensor cannot detect an abnormality of some kind. The abnormality which cannot be detected is an abnormality that the coils are short-circuited each other. Even if the coils are short-circuited each other, the output voltage of the bridge circuit is not fixed to the ground voltage or the power supply voltage. Moreover, the difference between the two outputs of the bridge circuit is not considerably large. Therefore, the disclosed structure cannot detect the abnormality that the coils are short-circuited each other.
If the controller cannot detect the above-mentioned abnormality, release of a clutch, which is establishing the connection between an electric motor for generating assist steering torque and a steering system in order to shift the mode to a complete manual steering state, is not permitted in spite of occurrence of an abnormality in a case where the torque sensor is applied to an electric power steering apparatus of a vehicle. What is worse, there arises a problem in that occurrence of the abnormality cannot be communicated to a driver by means of alarm display or the like.
In view of the foregoing, an object of the present invention is to provide a torque sensor which does not require a complicated neutral adjustment using a position adjustment mechanism, which is able to reduce cost and heat generation, which does not require a large space, which is able to improve reliability and which can be constituted by a single-system detection circuit.
Another object of the present invention is to provide a torque sensor which is capable of detecting even an abnormality that coil pairs are short-circuited.
To achieve the above objects, a torque sensor according to the present invention comprises: a control calculating portion for varying operation timing of a coil and sample holding timing for detecting torque; a storage portion for storing an initial value of each element of the torque sensor; a torque detecting portion detecting the torque based on sampling of transition voltage of the coil; and means for comparing the initial value in the storage portion with a current value of the each element of the torque sensor during sample holding in which the coil is not operated and detection of the torque is not performed, thereby detecting a failure of the each element of the torque sensor.
Further, there is provided a torque sensor which is used for an electric power steering apparatus which detects steering torque of a steering wheel and assists a rotation of a steering shaft integrally provided with the steering wheel with a motor. The torque sensor comprises: a torque detecting portion detecting the steering torque based on sampling of transition voltage of a coil; a control calculating portion for varying operation timing of the coil and sample holding timing; a storage portion for storing an initial value of each element of the torque sensor; and means for comparing the initial value in the storage portion with a current value of the each element of the torque sensor during sample holding in which the coil is not operated and detection of the torque is not performed, thereby detecting a failure of the each element of the torque sensor. Especially, the failure of the each element is detected in accordance with an A/D value after the sample holding at timing at which the coil is not operated, further, a neutral voltage is switched, and the failure of the each element of the torque sensor is detected in accordance with an A/D value after the neutral voltage has been switched according to the present invention.
The torque sensor according to the present invention has a structure that first and second rotating shafts disposed coaxially are connected to each other through a torsion bar; a cylindrical member made of a conductive and non-magnetic material is integrated with the second rotating shaft in a direction of rotation such that the cylindrical member covers the outer surface of the first rotating shaft; at least a covered portion of the first rotating shaft covered with the cylindrical member is made of a magnetic material; grooves extending in the axial direction are provided for the covered portion; windows are provided for the cylindrical member such that state of overlapping the grooves is changed in accordance with the relative rotational position with the first rotating shaft; and coils are disposed to surround the portion of the cylindrical member in which the windows are formed; electric resistors are disposed in series with the coils so that torque generated at each of the first and second rotating shafts is detected in accordance with transient voltage which is generated between the coil and the electric resistance when voltages each of which varies in the rectangular wave is applied to the coils. The non-magnetic material is a paramagnetic material or a partial diamagnetic material. The magnetic material is a ferromagnetic material The magnetic permeability of the non-magnetic material is similar to that of air. The foregoing magnetic permeability is lower than that of the magnetic material. The transition voltage is final voltage which is changed owning to supply of voltage which varies in the rectangular wave. The present invention has the structure that the coils are operated with the rectangular-wave voltages. Therefore, the intervals of supply of the rectangular-wave voltages are made to be in synchronization with sampling clocks of a controller portion to which an output of the torque sensor is produced. Thus, duration of electric current flow through each coil is considerably shortened and current consumption can be reduced. Also heat generation can be reduced. The rectangular-wave has an advantage that it can easily and accurately be generated with a small number of electronic parts as compared with sine wave.
Moreover, the present invention has a structure that each coil is operated by a method having different states of a stationary state and a transition state. Coil operation timing and sample holding timing are varied. When torque detection is not being performed, an operation different from that when a torque signal is being detected is performed. A comparison with an assembly initial value stored in the storage portion is made so that a failure of each element is detected. Moreover, timing at which each element is operated is combined to confirm all of the circuit portions. Thus, a single system circuit structure is realized.
Further, to achieve the above objects, another type of torque sensor according to the present invention comprises: directions in accordance with the torque; a pair of electric resistors each connected in series with the respective coil so as to detect the torque in accordance with transient voltage generated in portions in which the coils and the electric resistors are connected to each other; and switching means being capable of generating the transient voltage, the switching means being provided for each of the pair of the coils. One of the switching means is operated at timing at which the torque is not detected so that an abnormality of the pair of the coils is detected.
The above torque sensor has a structure that first and second rotating shafts disposed coaxially are connected to each other through a torsion bar; a cylindrical member made of a conductive and non-magnetic material is integrated with the second rotating shaft in a direction of rotation such that the cylindrical member covers the outer surface of the first rotating shaft; at least a covered portion of the first rotating shaft covered with the cylindrical member is made of a magnetic material; grooves extending in the axial direction are provided for the covered portion; two lines of windows are provided for the cylindrical member such that a state of overlapping the grooves is changed in accordance with the relative rotational position with respect the first rotating shaft; and a pair of coils are disposed to surround the portion of the cylindrical member in which the windows are formed; the pair of the coils and a pair of electric resistors are in series and individually connected to each other; a switching device is provided for each of the pair of the coils so that torque generated at the first and second rotating shafts is detected in accordance with transient voltages generated between the coils and the electric resistors when the switching devices are operated with the rectangular-wave voltages.
The non-magnetic material is a paramagnetic material or some of diamagnetic material. The magnetic material is a ferromagnetic material The magnetic permeability of the non-magnetic material is similar to that of air. The foregoing magnetic permeability is lower than that of the magnetic material. The transient voltage is a voltage which is changed owning to supply of voltage varying in the rectangular wave. Since the coils are operated with the rectangular-wave voltages, the intervals of supply of the rectangular-wave voltages is made to be in synchronization with sampling clocks of a controller portion to which an output of the torque sensor is produced. Thus, the duration of electric current flow through each coil is considerably shortened and current consumption can be reduced. Also heat generation can be reduced. The rectangular wave has an advantage that it can easily and accurately be generated with a small number of electronic parts as compared with sine wave. If a sample hold circuit for maintaining the transient voltage which is generated between the coils and the electric resistor is provided, the torque can be detected even if the transient voltage disappears in a short time.
A specific structure for detecting the pair of the coils may be a structure in which either of the switching means is operated at timing (between moments of time at which the two switching devices are operated with the rectangular-wave voltages in a specific example of the structure of the torque sensor) at which the torque is not detected. In accordance with an output voltage generated between the coil to which the other switching means correspond and the electric resistor, short circuit between the pair of the coils can be detected. That is, if one of the switching means is operated in a state in which the pair of the coils are not short-circuited, the voltage between the coil and the electric resistor must be the power supply voltage because no electric current is passed through the coil to which the other switching means correspond. In a state in which the pair of the coils are short-circuited, also an electric current is passed through the coil to which the other switching means corresponds when one of the switching means is operated. As a result, the voltage between the coil and the electric resistor is made to be lower than the power supply voltage.
Features and advantages of the invention will be evident from the following detailed description of the preferred embodiments described in conjunction with the attached drawings.